1. Field of the Invention
The present invention relates to an anti-fogging agent having excellent anti-fogging effects, an anti-fogging thermoplastic resin sheet comprising a coating film of the anti-fogging agent, and a formed article formed from the anti-fogging thermoplastic resin sheet by thermoforming.
2. Description of Related Art
Sheets produced by extruding various thermoplastic resins into films have been widely used as packaging containers for food and as packaging containers for other goods after being thermoformed into formed articles using vacuum forming machines, hot plate pressure forming machines, or a vacuum pressure forming machines. Among these resin sheets, resin sheets made of a transparent resin such as polystyrene, polyethylene terephthalate, or polypropylene are widely used as materials for lids of lunch boxes, side-dish containers or the like because of transparency. Such a thermoplastic resin sheet used as materials for lids of lunch boxes, side-dish containers or the like is coated with a surfactant such as sucrose fatty acid ester on one surface thereof in order to prevent fogging when the formed article is filled with the contents, and is then thermoformed into a formed article so that the surface subjected to an anti-fogging treatment constitutes the inner surface.
However, the formed articles coated with the surfactant are generally unsatisfactory in anti-fogging effects and also caused problems in that water droplets formed on the inner surface of the formed article, thereby obscuring the view of the contents and drastically reducing the commercial value when the formed article is filled with contents.
Particularly in the case in which the resin sheet is thermoformed using the hot plate pressure forming machine, since the sheet is heated while contacting the sheet surface coated with an anti-fogging agent with a hot plate, the anti-fogging agent peeled off and irregularities of the hot plate are transferred onto the resin sheet surface, thereby roughening the surface, and thus anti-fogging effects are drastically reduced. Also, there occurred problems in that a formed article suited for use in a refrigerated showcase is difficult to obtain because of poor anti-fogging effects in low temperature environment.
To solve this problem, Japanese Unexamined Patent Application, First Publication No. 2001-145983 discloses a technique of improving anti-fogging effects, particularly anti-fogging effects at low temperature in a food packaging formed article using an anti-fogging agent comprising a nonionic surfactant such as sucrose fatty acid ester and an organic acid alkali metal salt such as L-sodium tartrate (Japanese Unexamined Patent Application, First Publication No. 2001-145983 (see claims, paragraph number 0006)).
However, the technique disclosed in Japanese Unexamined Patent Application, First Publication No. 2001-145983 had a problem in that since the anti-fogging coating film formed on the sheet surface has poor strength, the anti-fogging coating film breaks at the drawn portion, and anti-fogging effects are drastically reduced when formed into a deep drawn formed article. Also, there occurred a problem in that when the resin sheet comprising the coating film of the anti-fogging agent is wound in a roll, the anti-fogging agent is likely to transfer to the surface (hereinafter referred to as “the opposite surface”) opposite the surface coated with the anti-fogging agent, and anti-fogging effects of the formed article are reduced.